Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14, which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
There are several prior art techniques for writing the servo sectors to the disk, including an external servo writer that writes the servo sectors by controlling the head internal to the disk drive using precise external positioning mechanics, such as a laser interferometer. Other techniques may involve self servo writing the servo sectors by servoing off of a seed pattern, or by propagating the servo sectors across the radius of the disk. In yet another technique, a media writer may simultaneously write the servo sectors to a number of disk surfaces, and then one or more of the servo written disks installed into a production disk drive. A similar technique may use a printing technique (e.g., stamping or etching technique) to servo write the disks which are then inserted into a production disk drive.
If ramp loading/unloading is used, it is desirable to locate the edge of the ramp with respect to the outer diameter of the disk as well as to locate the inner diameter (ID) crash stop in order to maximize the usable area of the disk. For example, an external servo writer or media writer that employs ramp loading/unloading may attempt to locate the edge of the ramp so that the servo sectors may be written up to the edge of the ramp, as well as locate the ID crash stop so that the servo sectors may be written up to the inner most boundary of the disk. Similarly, a seed servo writer may attempt to locate the edge of the ramp and/or the ID crash stop so that the seed tracks (e.g., spiral seed tracks) may be written up to the edge of the ramp across the disk to the inner diameter boundary defined by the ID crash stop. If a disk drive performs self servo writing of the servo sectors, again it is desirable to write the servo sectors up to the edge of the ramp to the inner diameter boundary. If the servo sectors are written (or printed) and then the servo written disk installed into a disk drive (including to install the edge of a ramp over the edge of the disk), it is desirable to locate the edge of the ramp to determine the first usable servo track as well as locate the inner diameter boundary (based on the ID crash stop) and thereby format the disk with corresponding data tracks.
Prior art techniques for locating the outer diameter and inner diameter boundaries of a disk include evaluating a back electromotive force (BEMF) voltage generated by the VCM. However, the VCM's BEMF voltage is a noisy signal that requires a margin from the detected disk boundaries, thereby reducing the overall usable area of the disk surface. In addition, the VCM BEMF voltage cannot distinguish between the actuator arms for each disk surface and therefore the VCM BEMF voltage cannot be used to identify the disk boundaries for each individual disk surface.